Line driver-line receiver circuits are used for example in digital or pulse communications and typically employ a differential transmission line to couple differential, i.e., positive and negative, outputs of a line driver (sometimes called a line transmitter) and corresponding differential inputs of the line receiver. Noise can often be a problem in such configurations. For example, if all differential transmission pairs bundled in a cable are not coupled, signals from the open or unterminated line drivers can adversely impact the desired signaling on another differential transmission pair in the cable, especially on the open line receivers. Such an open transmission line can act as an antenna, picking up noise from adjacent transmission line pairs in the cable and elsewhere, and delivering that noise to the open line receivers. This noise can be detected as invalid data.
The present invention overcomes this and other types of noise detection using bias circuitry. First bias circuitry is coupled across the first and second inputs of a line receiver. This first bias circuitry provides sufficient bias to protect the line receiver from being affected by incoming noise, e.g., noise resulting from a line driver not being coupled to the transmission line. However, when the line driver is coupled to the transmission line, second bias circuitry is engaged to compensate for undesirable effects on data signals received in the first and second inputs of the line receiver caused by the first bias circuit, e.g., asymmetric data waveforms. Thus, the first and second bias circuitry offset one another, and preferably, cancel out the effect of the other.
In a specific, non-limiting example embodiment, the first bias circuitry includes a first resistor coupled to a first voltage and to a first input of the line receiver, and a second resistor coupled to a second input of the line receiver and to ground. The second bias circuitry includes a third resistor coupled between a second voltage and a second output of the line driver, and a fourth resistor coupled between the first output of the line driver and ground. The first and third resistors have substantially the same value. The second and fourth resistors have substantially the same value. The first and second voltages are substantially the same.
An example method in accordance with the present invention may be used in conjunction with a line driver having first and second outputs coupled through a differential transmission line to first and second inputs of a line receiver. A first bias is provided between the first and second inputs of the line receiver. A second bias is applied between the first and second outputs of the line driver. As a result, the first and second biases substantially offset, and preferably, cancel each other. If the differential transmission line is not coupled to a line driver, the second bias associated with that line driver is not applied permitting the first bias to provide a relatively high noise margin. This high noise margin is advantageous, for example, when the differential transmission line is one of plural differential transmission lines coupled to plural line receivers.
A non-limiting, example application of the present invention is in a radio base station. The base station includes a first unit that is configured to receive multiple transceiver units and is coupled to a cable that contains multiple corresponding differential transmission lines. Each transceiver unit includes a line driver coupled to one of the differential transmission lines. Each transceiver unit is associated with a bias circuit configured to apply a bias voltage across the first and second outputs of its associated line driver. The base station also includes a switch unit coupled to the cable which includes plural line receivers. Each line receiver includes first and second inputs coupled to one of the differential transmission lines in the cable. Each line receiver also includes another bias circuit that is configured to apply another bias across the first and second inputs. The line driver of each transceiver unit is coupled by one of the differential transmission lines to one of the line receivers. The bias circuit associated with the line driver compensates for the bias circuit associated with the line receiver. The bias circuit associated with the line receiver provides sufficient bias to protect the line receiver from incoming noise if a line driver is not coupled to the other end of the differential transmission line.
The present invention provides a number of advantages. First, and perhaps most significant, because a relatively high bias voltage can be applied across the inputs of the line receiver, a fairly high level of noise can be tolerated on the differential transmission lines without any transfer of invalid data from an open transmission line. Second, the high bias provided by the first bias circuit does not negatively impact the normal operation of the line circuit when a line driver is attached and transmitting data. As explained below, asymmetries in the data waveforms received on the positive and negative inputs of the line receiver are effectively eliminated by the second biasing circuit which offsets the bias provided by the first bias circuit. Third, the present invention is easy to implement with just a few simple resistors. Fourth, the invention is cost effective because there is no need to use shielded cables, transmission lines, and connectors that would otherwise be needed to avoid picking up noise and other distortion on open-ended transmission lines.